We are continuing our search for monoclonal antibodies that are directed against nuclear antigens whose levels change significantly during the G1 period of primary cultures of chick embryo fibroblasts. We have improved the sensitivity of the ELISA assay. Nevertheless, examination of several thousand hybridoma supernatant fluids has not yet yielded a positive result. As an offshoot of this project, we have begun using monoclonal antibodies to look for differences in the levels of nuclear proteins among the various types of specialized cells that make up the chick embryo kidney. Two very interesting monoclonal antibodies have been found, and efforts are now being made to identify the antigens and to discover their functions. Efforts are continuing to ascertain whether the mRNAs that code for ribosomal proteins are translated with difficulty in mammalian cells in culture and in vivo as they are in chick embryo fibroblasts. Evidence has already been accumulated to show that this is the case for HeLa and 3T3 cells. Finally, to learn about the mechanism of action of insulin, the impact of the hormone on phosphatidylinositol metabolism is being studied in chick embryo fibroblasts, and comparisons are being made of the effects of insulin and phorbol esters on the cultured cells. (N)